1. FIELD OF THE INVENTION
The present invention relates to a photographic silver halide emulsion, a photographic element and a method of forming color photographic images, and more particularly to color photographic materials and forming images using a yellow color forming coupler.
2. DESCRIPTION OF THE PRIOR ART
It is well known that color forming couplers couple with oxidation products of primary aromatic amino developing agents to form color photographic images. Most conventional yellow color forming couplers are four-equivalent couplers and they require four moles of exposed silver halide as oxidizing agents to form one mole of azomethine dye. An incorporation of a large amount of silver halide into light-sensitive layers leads to some disadvantages such as an increase in light scattering in the emulsion layers and consequently, a decrease in the sharpness of the images formed, and in addition, an increase in processing time of light-sensitive materials due to an increase in thickness of the emulsion layers. Furthermore, the formation of dyes with these couplers is not completed during color development and thus it is necessary to use strong oxidizing agents in subsequent processing steps.
In order to improve these defects, two-equivalent yellow color forming couplers have been provided, which require only two moles of exposed silver halide to form one mole of azomethine dye.
The two-equivalent yellow color forming couplers have, in general, chemical structures in which one of the hydrogen atoms of the active methylene group is substituted with a splittable atom or group. Examples of such splittable atoms or groups are a fluorine atom as described in U.S. Pat. No. 3,277,155, a phenoxy group as described in U.S. Pat. No. 3,408,194, an acyloxy group as described in U.S. Pat. No. 3,447,928, a sulfoxy group as described in U.S. Pat. No. 3,415,652, and a group having saccharin structure as described in German Offenlegungsschrift 2,057,941.
However, these couplers are not sufficient for use in color photography because they are accompanied by either disadvantages in that the coupling reactivity is insufficient, in that a marked color fog is produced, in that the couplers per se are unstable and their coupling activities decrease or color stain occurs in the light-sensitive materials during storage, in that the yellow color images formed are unstable or in that the preparation of the couplers is quite difficult.
As couplers which improve these defects, yellow color forming couplers having a splittable group derived from imide compounds are described in Japanese Patent Application laid open 29432/73, U.S. Patent Applications Ser. Nos. 235,937, filed Mar. 20, 1972, and 319,806, filed Dec. 29, 1972.
However, in color photographic light-sensitive materials containing the couplers described in these patent specifications, it is not easy to completely remove developed silver which is formed during a color developing step and is undesirable in the final photographs in bleach and fixing steps or in a blix step in which both bleach and fixing steps are simultaneously carried out in a mono bath. Thus it is very difficult to reduce the processing time. Also, when the developed silver is not completely removed, the color reproduction of the color images obtained is adversely affected and transparency of the images is reduced. Thus, provision of two-equivalent yellow color forming couplers which have a good silver bleaching property has been desired.